The lithium secondary battery using a non-water based electrolyte solution has been already put to practical use in the related fields of information-communication devices such as personal computers and cellular phones because this type of battery can give high-voltage and high-energy densities with a reduced size and weight. Further, the lithium secondary battery is expected to find further applications as a power supply mounted in an electric vehicle or a hybrid electric vehicle in order to accommodate the problems of resources and environments.
Generally, a non-water based lithium secondary battery is constituted by combining a lithium transition metal composite oxide as its positive electrode active material, a carbon material as its negative electrode active material, and an non-aqueous electrolyte solution obtained by dissolving lithium salt in an organic solvent.
There is another type of water based lithium secondary battery available that uses an aqueous solution as its electrolyte solution. The water based lithium secondary battery does not need dry environments in its manufacturing process in contrast to the non-water based lithium secondary battery and so can reduce the manufacturing costs significantly. Further, the aqueous electrolyte solution generally has higher conductivity than the non-water based electrolyte solution, so that the water based lithium secondary battery has an advantage of lower internal resistance than that of the non-water based lithium secondary battery.
On the other hand, however, the water based lithium secondary battery is required to be used in a potential region in which water will not electrolyzed and so has smaller electromotive force than the non-water based lithium secondary battery.
As calculated from the electrolysis voltage of water, the electromotive force is about 1.2V at the maximum but, in reality, expected to be about 2V at the maximum because an over-voltage is necessary to give rise to a gas in electrolysis.
Thus, the water based lithium secondary battery can reduce the costs and the internal resistance at the cost of high voltage densities, that is, high energy densities. Therefore, the water based lithium secondary battery is not suitable for use in the instruments such as cellular phones mainly required to be high in energy density, that is, light and small but may be expected to be suited for use as a power supply in the electric vehicles and hybrid electric vehicles and, furthermore, as a household-purpose dispersed power supply, which are relatively put importance on costs and required to be large scaled.
What is important in constitution of the water based lithium secondary battery is to use an active material that can reversibly absorb and desorb a lot of lithium stably in an aqueous solution and in a potential region in which neither oxygen nor hydrogen will be produced in the electrolysis of water, that is, an active material that can give a large capacity in a specific potential region.
Further, the electrolyte solution to be used should desirably be neutral through alkaline. This is because generally the Li containing oxides mainly used as the active material lack in stability in a strongly acidic aqueous solution and, further, a lot of H+ions in the acid electrolyte solution may possibly inhibit the rocking chair reaction of pure Li+ions.
So far, as the positive electrode active material used in water based lithium secondary batteries, LiMn2O4, LiFePO4, etc. have been proposed (see Patent Documents 1 to 3). These positive electrode active materials are comparatively stable in an aqueous solution and can realize a comparatively large capacity.
On the other hand, as the negative electrode active material, manganese oxides, iron oxides, iron oxide hydroxides, vanadium oxides, titanium based polyanion compounds, etc. have been proposed (see Patent Documents 3 to 8).
Among these, the vanadium oxides are considered to be most promising as the negative electrode active material used in the water based lithium secondary battery. In particular, LiV2O4 having the spinel structure can realize a comparatively large capacity in an aqueous electrolyte solution because it can stably bring about reversible insertion and absorption of Li in the potential region in which water will not be electrolyzed.
In such a water based lithium secondary battery, its positive and negative electrodes are generally formed in a condition where the positive and negative electrode active materials are supported on current collectors made of aluminum or nickel etc.
Further, as the electrolyte solution, it has been proposed to use an aqueous solution of a lithium salt such as lithium sulfate, lithium chloride, lithium acetate, or lithium nitrate (see Patent Document 9 and Non-patent Document 1).
Patent Document 1: Published Japanese translation of PCT international application No. Hei 9-508490
Patent Document 2: Japanese Patent Application Laid-Open No. 2002-260722
Patent Document 3: Japanese Patent Application Laid-Open No. 2002-110221
Patent Document 4: Japanese Patent Application Laid-Open No. 2000-340256
Patent Document 5: Japanese Patent Application Laid-Open No. 2000-77073
Patent Document 6: Japanese Patent Application Laid-Open No. 2001-102086
Patent Document 8: Japanese Patent Application Laid-Open No. 2003-17057
Non-patent Document 1: H. Wang et al. “Electrochimica Acta”, Britain, issued on Feb. 15, 2007, Vol. 52, No. 9, p. 3280-3285